Should a student's grade level be based entirely on how old he is or at least partially on how skilled he is? This is the fundamental question underlying the debate over social promotion ï¿½ the practice of moving students to the next grade regardless of whether they have acquired the minimal skills covered in the previous grade.

Advocates of social promotion suggest that it is best to group students by age rather than by skill. Students who are held back a grade are separated from their age peers and, the argument goes, this social disruption harms them academically. Opponents of social promotion favor requiring students to demonstrate minimal skills on a standardized test before they receive automatic promotion to the next grade.

Until now the bulk of the research favored social promotion. But this previous research is that it was never entirely clear whether retained students did worse because they were retained or because whatever caused them to be retained led to worse outcomes.

When teachers decide that one student should be retained while another demographically similar student should be promoted, they probably know something about those students that suggests the promoted student has better prospects than the retained student.

In a new study we conducted for the Manhattan Institute, we find that holding back low- performing students helps them academically. We examined a policy in Florida that required third-grade students to perform at a certain level on the state's reading test before they could be promoted. Students who performed below that level and repeated third grade made significantly greater academic progress than similar students who were promoted.

The benefit of being retained grew so that by the end of the second year the retained students entered fifth grade knowing more than the promoted students did leaving fifth grade.

Of course, the key question is how do we know that we are comparing similar students when earlier researchers had so much difficulty making apple-to-apple comparisons? We are helped by the fact that retention decisions in Florida were based on the adoption of an objective test requirement rather than educator discretion. This allowed us to pursue two strategies.

First, we could compare the academic progress made by low-performing students the first year the requirement was adopted to the progress of similarly low-performing students the year before the policy was put in place.

Second, we could compare the academic progress of students who were barely above the minimum test score (almost all of whom were promoted) to the progress of students who were barely below the required test score (most of whom were retained).

Whichever way we looked at it, the result was the same: Retained low-performing students made significantly greater academic progress than promoted low-performing students.

Of course, this study does not definitively prove that test-based retention is beneficial. For one thing, researchers using similar methods analyzing a similar program in Chicago found that retained students fared no better or slightly worse than promoted students.

The point is that we have strong evidence from Florida that test-based promotion requirements can significantly enhance the achievement of low-performing students. If those positive results continue and can be replicated, we may have to rethink the widespread idea that students have to be grouped in grade levels by age rather than by skill level.

Perhaps school systems will be motivated to ensure that students acquire the required skills if they can't simply pass students along, regardless of their achievement.